With our aging population, total knee arthroplasty is performed with increasing frequency. Although the surgery is generally successful, many patients have persistent pain and disability. Traditional risk assessment tools have focused on single organ systems. Our team has found that mobility, assessed by the Mobility Assessment Tool short form (MAT-sf), is a simple and accurate method to predict postoperative outcomes, including length of stay, postoperative complications, and nursing home placement for older patients. Prehabilitation is the process of enhancing a person's functional capacity to withstand an incoming stressor. Although multiple studies have tested prehabilitation before joint replacement surgery, results have been mixed. We hypothesize that patients with limited mobility are most likely to benefit from prehabilitation. We plan to use individualized aquatic exercise as a prehabilitation tool; the resistance of water strengthens muscle and increases energy expenditure without stressing weight-bearing joints. The aims of this exploratory R03 study are: 1) To evaluate the feasibility of prehabilitation using 6-8 weeks of aquatic exercise in 40 older adults scheduled for total knee arthroplasty for osteoarthritis; 2) To examine the effects of 6-8 weeks of aquatic exercise on (a) mobility, pain, stiffness, physical function, cognitive function and depression; (b) body composition measured by dual-energy X-ray absorptiometry (DXA); and (c) inflammatory markers, including CRP, IL-6 and TNF-a; and 3) To estimate the effect of prehabilitation on postoperative outcomes. We will screen patients in the Preoperative Assessment Clinic and enroll patients who have decreased mobility, measured by MAT-sf. Patients will be randomized into either a prehabilitation group or a usual care group. All will be extensively assessed regarding pain, stiffness, and physical function, depression, balance and cognitive function using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Geriatric Depression Scale Short Form (GDS-SF), the Short Physical Performance Battery (SPPB), Berg Balance Scale, and Montreal Cognitive Assessment (MoCA). Serum inflammatory markers including CRP, IL-6 and TNF-alpha and body composition, using dual energy X-ray absorptiometry (DXA), will be assessed at the baseline. The prehabilitation group will participate in 6-8 weeks of individualized aquatic exercise in a heated pool (60 min/session, 3 times per week). All participants will be reassessed immediately before surgery and 4 weeks after the surgery using WOMAC, GDS-SF, SPPB, Berg Balance Scale, MoCA and MAT-sf. Serum inflammatory markers and body composition will be reassessed at the same time points. The primary outcomes of interest are postoperative complications, length of stay, ICU length of stay, and institutionalization. If successful, we will have sound pilot data for several critical health outcomes with which to support an external proposal for a larger-scale study. This study and the career development activities described will help the applicant to become an independent researcher in geriatric perioperative medicine.